I'm trying to get a really old laptop (Mac Powerbook 170) running again but
its main battery seems to have died. I could open up the old battery pack
and replace the cells but I was wondering if there was a simpler solution.

Does anyone know whether attaching a capacitor between the terminals instead
of a battery and using the AC adapter for power would work...?

Unplugging the battery and running it directly from its AC adapter didn't do
anything, so I was wondering whether it needed the battery to complete the
power circuit.

To test this I thought connecting a capacitor (and perhaps a small resistor)
across the battery terminal would have similar characteristics to a flat
battery. Wouldn't the capacitor then come up to the correct voltage as the
laptop tries to charge it?